[U-Boot] Fastboot behaviour with sparse images

[Maxime Ripard <maxime.ripard@free-electrons.com>]

Hi Colin,

On Tue, Oct 27, 2015 at 02:00:24PM -0700, Colin Cross wrote:
> On Monday, October 12, 2015 at 6:43:37 AM UTC-7, Maxime Ripard wrote:
> >
> > Hi, 
> >
> > I'm currently writing the support in U-Boot for NAND-backed devices 
> > using fastboot [1], and that work derived a bit to supporting the 
> > sparse images. 
> >
> > For "regular" images that are being stored, we expect a pair of 
> > download and flash commands. Simple. 
> >
> > Things start to get a bit more complex with sparse images that have 
> > been split because of a max-download-size lower than the actual image 
> > size. 
> >
> > Here, from what I could gather from various random blog posts, the 
> > fastboot client implementation and dumping a few USB sessions, the 
> > client simply creates several download / flash pairs, always on the 
> > same partition, without any way to distinct that from several 
> > subsequent writes issued by the user. 
> >
> > So, I'm guessing that the expectation is that the bootloader 
> > implementation should store the last offset it wrote to, and simple 
> > resume from there if the partition names in the flash commands are the 
> > same, which would prevent two subsequent write on the same partition 
> > by any client. Am I right? 
> >
> 
> No, each blob passed to the bootloader will begin with a sparse "skip" 
> chunk that will seek to the correct place to resume writing.  The 
> bootloader shouldn't need to store any metadata across commands.  Just read 
> in the blob from the data command, then write it out using a port of the 
> Apache-licensed libsparse during the flash command.

Oh, so that's how it works. Great. I guess however that you still need
to scan out the area you skip for bad blocks to account them in the
offset calculation as well then (when you're using NAND).

> > A related question is when should we erase the NAND partition? Only 
> > when doing fastboot erase, or also when doing fastboot write (which, 
> > combined with the issue raised above, would also mean that we don't 
> > want to do an erase on the whole partition everytime there's a flash 
> > command on it).
> 
> Fastboot should send an erase command before every sequence of writes. 
>  Erase the whole partition on the erase command, and don't erase anything 
> on the flash command.

Ack.

Thanks!
Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com
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